Gearbox: Cracked or broken tooth

Cracked or broken tooth

Symptoms: High 1X radial / Gear natural frequency / 1X sidebands around the gearmesh frequency

A cracked or broken tooth will generate a high amplitude peak at the turning speed of that gear, and it will cause the gear natural frequency to be excited. There will be 1X sidebands around the gear natural frequency (1X of the gear with the damaged tooth).

There is no way to know ahead of time what the gear natural frequency is. You must watch for a new peak in the spectrum that has a broad base and sidebands as described.

Spectrum: Look for the gear natural frequency peak with sidebands.  Spectrum analysis is not the best diagnostic tool.

Time waveform: If you view a time waveform that contains approximately 6-10 revolutions of the gear in question then the vibration change as the tooth comes into mesh should be clear to see.  Circle plots and time synchronous averaging are also very useful analysis tools.

Wear particle analysis: Metal from the gear should be detected in wear particle analysis tests (and hopefully oil analysis tests as well).